In a film forming process on a semiconductor wafer (hereinafter refer to as wafer), a liquid source can be used. For example, when forming a tantalum oxide film which is used as DRAM's capacitor film, a liquid source, PET (Tantalum Pentaethoxide: Ta(OC2H5)5) is used. When carrying out the film forming process, PET is transferred from a liquid source tank to a vaporizer through a piping, and vaporized therein. The PET flow rate to the vaporizer is controlled by a mass flow controller, which is a liquid flow controller, installed at the piping. In addition to vaporized PET, oxygen gas is supplied into a processing vessel where a wafer is loaded, to perform a film forming process.
When replacing the liquid source tank or the vaporizer, they are separated from the piping. Since PET solidifies when it comes in contact with moistures, the inside of the piping must be thoroughly washed by a washing fluid, such as alcohol to completely remove PET prior to exposing the piping to the atmosphere. Further, after coupling the piping, air must be completely forced out from the inside of the piping. If cleaning is not performed sufficiently, residual PET inside the piping reacts with the moistures in the air and solidifies, thereby attaching itself to the inside of the piping. Still further, if the washing fluid's water content is high, PET reacts with the moistures in the washing fluid and solidifies similarly. The mass flow controller gauges a fluid flow rate, based on the difference of heat transfers from the fluid at an upstream and a downstream point of a flow sensor. Therefore, if solidified PET is attached to the inside of the mass flow controller, the heat transfer is altered and reading (sensing) error occurs so that the PET flow rate may be several times higher than a rate set to the mass flow controller.
If PET flow rate deviates from a set flow rate, it is possible that the capacity of a capacitor cannot be obtained as planned. If the flow rate of PET is greater than a set flow rate, the amount of PET consumption becomes so high that not only the liquid source tank needs to be replaced frequently, the cleaning cycle of the processing vessel becomes short as well. Further, if the reading (sensing) accuracy of the mass flow controller declines, the integrating flow rate management by computer cannot be properly performed.
In order to prevent such problem, the mass flow controller is periodically separated from the piping and tested. This test is conducted in the following manner: alcohol is sent into the mass flow controller and the alcohol amount is measured by a mess cylinder, and the reading from the cylinder is compared with the flow rate set to the mass flow controller. However, the amount of PET used during film forming is small and the type of mass flow controller used is also for low flow rate applications, it is difficult to measure precisely the flow amount by using the mess cylinder.
Before separating the mass flow controller from the piping, the inside of the piping and the mass flow controller are cleaned. However, if cleaning is not performed sufficiently, residual PET inside the mass flow controller reacts with moistures in the air and becomes solidified. Further, if the air that gets into the piping is not completely removed after the testing is completed and the mass flow controller is reinstalled in the piping, there is a concern for PET solidification when it is sent through the piping. Since such solidified PET becomes, so to speak, similar to ceramic, it is practically impossible to clean and remove the solidified PET, and the mass flow controller must be replaced. Namely, there is a concern that malfunctioning of a mass flow controller can be attributed to the very process employed to confirm the mass flow controller's proper operation.